Structural basis for catalysis and ubiquitin recognition by the severe acute respiratory syndrome coronavirus papain-like protease.

Papain-like protease (PLpro) is one of two cysteine proteases involved in the proteolytic processing of the polyproteins of Severe acute respiratory syndrome coronavirus (SARS-CoV). PLpro also shows significant in vitro deubiquitinating and de-ISGylating activities, although the detailed mechanism is still unclear. Here, the crystal structure of SARS-CoV PLpro C112S mutant in complex with ubiquitin (Ub) is reported at 1.4 Šresolution. The Ub core makes mostly hydrophilic interactions with PLpro, while the Leu-Arg-Gly-Gly C-terminus of Ub is located in the catalytic cleft of PLpro, mimicking the P4-P1 residues and providing the first atomic insights into its catalysis. One of the O atoms of the C-terminal Gly residue of Ub is located in the oxyanion hole consisting of the main-chain amides of residues 112 and 113. Mutations of residues in the PLpro-Ub interface lead to reduced catalytic activity, confirming their importance for Ub binding and/or catalysis. The structure also revealed an N-cyclohexyl-2-aminethanesulfonic acid molecule near the catalytic triad, and kinetic studies suggest that this binding site is also used by other PLpro inhibitors. Overall, the structure provides a foundation for understanding the molecular basis of coronaviral PLpro catalysis.

Differential domain structure stability of the severe acute respiratory syndrome coronavirus papain-like protease.

Papain-like protease (PLpro) from severe acute respiratory syndrome (SARS) coronavirus is one of the two proteases involved in the proteolytic processing of the virion polyproteins. In addition, PLpro shows significant in vitro deubiquitinating and de-ISGylating activities. All these findings demonstrated the multifunctional nature of the PLpro. Here we report the sensitivity of PLpro to denaturant urea. An increase in urea concentration induced a reversible biphasic unfolding of the enzyme. Differently, the unfolding of the catalytic triad region located within the palm and thumb domains followed a monophasic unfolding curve. Further observations suggest that the zinc-binding domain may start to unfold during the first transition. An 80% lost of its enzymatic activity at a urea concentration lower than 1M showed a close correlation with unfolding of the zinc-binding domain. The enzyme was also characterized in terms of hydrophobicity and size-and-shape distribution. We have demonstrated that PLpro displayed differential domain structure stability and molten globule state in its folding. These studies will not only assist in our understanding of the folding of this viral enzyme, but also that of other deubiquitinating enzymes with a similar scaffold.

Quinazolin-2,4-dione-Based Hydroxamic Acids as Selective Histone Deacetylase-6 Inhibitors for Treatment of Non-Small Cell Lung Cancer.

We designed and synthesized quinazolin-2,4-dione-based hydroxamic acids to serve as selective competitive inhibitors of histone deacetylase-6 (HDAC6). The most potent and selective compound, 3d (IC50, 4 nM, HDAC6; IC50 > 10 µM, HDAC1), substantially increased acetylation of α-tubulin instead of histones in the lung cancer cell line, LL2. Paclitaxel in combination with 3d had a synergistic anticancer effect on reduction of programmed death-ligand 1 expression in LL/2 cells. When given orally, 3d was mainly found to locate in the liver and lungs, at a concentration 18- to 70-fold greater, respectively, than in plasma. As an orally active HDAC6 inhibitor, 3d (20 mg/kg) potentiated paclitaxel antitumor activity (percentage tumor growth inhibition, 67.5%) in a xenograft syngeneic non-small cell lung cancer mouse model.

Synthesis, molecular structure and characterization of allylic derivatives of 6-amino-3-methyl-1,2,4-triazolo[3,4-f][1,2,4]-triazin-8(7H)-one.

1-Allyl- (2) and 7-allyl-6-amino-3-methyl-1,2,4-triazolo[3,4-f][1,2,4]triazin-8(7H)-one (3) were obtained via the 18-crown-6-ether catalyzed room temperature reaction of 6-amino-3-methyl-1,2,4-triazolo[3,4-f][1,2,4]triazin-8(7H)-one (1) with potassium carbonate and allyl bromide in dry acetone. The structures of these two derivatives were verified by 2D-NMR measurements, including gHSQC and gHMBC measurements. The minor compound 2 may possess aromatic character. A single crystal X-ray diffraction experiment indicated that the major compound 3 crystallizes from dimethyl sulfoxide in the monoclinic space group P2(1)/n and its molecular structure includes an attached dimethylsulfoxide molecule, resulting in the molecular formula C(10)H(16)N(6)O(2)S. Molecular structures of 3 are linked by extensive intermolecular N-H...N hydrogen bonding [graph set C(1)(1)(7)]. 1 Each molecule is attached to the dimethyl sulfoxide oxygen via N-H...O intermolecular hydrogen bonding. The structure is further stabilized by pi-pi stacking interactions.